The present invention relates to a printer or similar image forming apparatus and, more particularly, to an image forming apparatus which allows each image component of particular color to be transferred to a medium in accurate register with the others by preventing image data of respective colors from being written at different timings and maintaining an image transfer position on the medium constant.
It is a common practice with an analog black-and-white copier to uniformly charge a photoconductive element being rotated by a corona discharger, exposes the charged element imagewise by an exposing device to electrostatically form a latent image, develop the latent image by a developing unit storing a black toner, and then transfer the resulting toner image to a sheet, In the case of an analog color copier, the uniformly charged surface of the photoconductive element is sequentially exposed to color components constituting a document image to form corresponding latent images. These latent images are each developed by a developer of associated color, and the resulting toner images are transferred to a sheet one over the other. An encoder is mounted on the drive shaft of the photoconductive element to generate basic pulses in association with the rotation of the shaft. In response to the basic pulses, a controller controls the sequence of various sections included in the copier by, for example, incrementing a counter having a timer function.
In a digital black-and-white copier, an image scanner scans a document image to generate digital image data. A photoconductive element is rotated and uniformly charged by a corona discharger. An optical writing unit modulates a laser beam by image data fed from the image scanner and writes the modulated image data on the charged photoconductive element by scanning it in the main scanning direction. As a result, an electrostatic latent image is formed on the photoconductive element. The latent image is developed by a developing unit storing a black toner and then transferred to a sheet. On the other hand, in a digital color copier, an image scanner reads color components constituting a document image while digitizing the resulting image data. An optical writing unit sequentially modulates a laser beam by the image data of respective colors fed from the image scanner so as to write them on a photoconductive element by scanning it in the main scanning direction. The resulting latent images of respective colors are each developed by associated one of toners of different colors. Toner images produced by such a procedure are sequentially transferred to a sheet one over the other to complete a single color image.
Japanese Patent Laid-Open Publication No. 242471/1987 discloses a laser beam printer having a plurality of laser units, and a plurality of photoconductive elements each being associated with one of the laser units. Laser beams issuing from the laser units are manipulated by respective polygonal rotary members which are driven by exclusive motors, thereby scanning the associated photoconductive elements to record a color image. In this laser beam printer, the rotating surfaces of each polygonal member are positioned perpendicular to the polar direction of the associated motor. Write control means controls the writing of image data on each photoconductive element in response to predetermined signals of predetermined period derived from one of the polygonal members.
On the other hand, Japanese Patent Laid-Open Publication No. 70265/1988 teaches a control device for a color copier. The control device generates a reference signal in synchronism with the rotation of a photoconductive element so as to define a reference position for starting forming a latent image. When the photoconductive element reaches a predetermined angular position as determined on the basis of the reference signal, an optical scanning mechanism is started up to maintain the element and the mechanism in a predetermined synchronous relation with respect to the start-up timing.
Assume that the sequence control system described in relation to the analog copier is applied to the digital copier. Then, since the basic pulses are not synchronous to the main scanning lines of the laser beam, the timing for writing read image data is apt to deviate by one line at maximum. Such a deviation would shift the position where image data should be written. Specifically, when a plurality of toner images sequentially formed on the photoconductive element are to be transferred to a sheet one above the other, the sheet is driven at a timing synchronous to the rotation of the element and, therefore, the images written on the element at deviated positions are directly transferred to the sheet. As a result, the toner images are brought out of register on the sheet. This is not critical as far as the black-and-white copier is concerned. However, when it comes to a color copier, the toner images of respective colors are brought out of register due to the positional deviation of the image data, critically degrading the quality of the resulting color image.
The laser beam printer of the type controlling the writing of image data on all of the photoconductive elements in synchronism with predetermined signals derived from one polygonal member as stated above has the following problem. Since the signals of predetermined period derived from one polygonal member involve an error due to irregularities in the rotation of the member, an error is introduced in the writing of image data to cause the image components of respective colors to deviate. This also lowers the quality of the resulting color image.
Further, the control device for a color copier described above has a drawback that the synchronous relation between the photoconductive element and the optical scanning mechanism is affected by an error occurring in the drive of the scanning mechanism. This also introduces an error in the writing of image data and thereby degrades the quality of a color image.